As disclosed in U.S. Pat. Nos. 5,088,863 and 5,230,593, two types of drills for chip removing machining are previously known, one of the types having straight cutting edges, and the other of the types having curved cutting edges.
Generally, in drills having straight cutting edges, each chip flute is relatively shallow in the sense that a curve defining the concave cross-sectional shape of the flute surface has a comparatively large radius of curvature. This means that chip jamming easily arises in the space behind the cutting edge confined between the flute surface and the hole wall. This results in vibrations in the drill, which in turn may result in the drill breaking. In drills having curved cutting edges, the chip flute may be formed with a greater depth, i.e. a smaller radius of curvature. Thereby, the tendency for chip jamming in the flute is less than for drills having straight cutting edges. However, a curved cutting edge entails a number of disadvantages, such as the grinding of the cutting edge, the sharpness thereof, etc.
Thus, according to the known technique accounted for above, the design of the cutting edge also controls the design of the chip flute.